Up until recently, the electronic motion in atoms and molecules, which typically occurs within tens to hundreds of attoseconds (1as = 10-18s), remained unexplored due to the fundamental limit in time resolution. This limit fell with the invention of the so-called ``attosecond streak camera''. The camera makes use of the high harmonic generation process which converts a driving near-infrared (NIR) femtosecond pulse into coherent extreme ultraviolet (XUV) bursts, at least one order of magnitude shorter than can be produced by conventional laser systems. An attosecond XUV burst sets an atomic electron in motion while the same driving NIR pulse, after a carefully monitored time delay, is used as a strobe.
With the attosecond streaking technique, it has become possible to determine the time delay between subjecting an atom to a short XUV pulse and subsequent emission of the photoelectron. This observation opened up a question as to when does atomic photoionization actually begin. We address this question by solving the time dependent Schrödinger equation and by carefully examining the time evolution of the photoelectron wave packet. In this way we establish the apparent ``time zero'' when the photoelectron leaves the atom. At the same time, we provide a stationary treatment to the photoionization process and connect the observed time delay with the quantum phase of the dipole transition matrix element, the energy dependence of which defines the emission timing.
An example of such an analysis is shown in the figure where we consider the valence shell photoionization of neon driven by an XUV pulse. The crest position of the electron wave packet after the end of the XUV pulse is fitted with the straight line which corresponds to the free propagation. In the inset, extrapolation of the free propagation inside the atom is shown. The opposite signs of the time delays t0(2s)<0 and t0(2p)<0 is related to the energy dependence of the p and d scattering phases which is governed by the Levinson-Seaton theorem.
We made another useful application of the attosecond time delay measuring technique in the case of double photoionization (DPI) of helium. We demonstrated that an measurement can distinguish between the two leading mechanisms of DPI: the fast shake-off (SO) and the slow knock-out (KO) processes. The SO mechanism is driven by a fast rearrangement of the atomic core after departure of the primary photoelectron. The KO mechanism involves repeated interaction of the primary photoelectron with the remaining electron bound to the singly charged ion.
A. S. Kheifets, I. A. Ivanov and Igor Bray
Timing analysis of two-electron photoemission
J. Phys. B 44, 101003, 2011
A. S. Kheifets and I. A. Ivanov
Delay in atomic photoionization
Phys. Rev. Lett 105, 233002, 2010